With the development of the semiconductor fabrication into the 40 nm technology node and below, the reduction in both the size of contact holes and the interval between contact holes has imposed great challenges to the photolithography/etching processes during the formation of contact holes. It has become an important issue to both ensure a good exposure result and prevent a short circuit of the contact holes after etching.
FIGS. 1 to 8 show a method of forming contact hole of the prior art. Reference is first made to FIG. 1 which illustrates a semiconductor substrate 10 on which a first dielectric layer 11, a second dielectric layer 12, a third dielectric layer 13 (which is an advanced pattern film (APF)), a dielectric anti-reflective coating (DARC) layer 14, a first bottom anti-reflective coating (BARC) layer 15 and a first photoresist layer 16 are formed. A first opening is formed in the first photoresist layer 16, exposing the underlying first BARC layer 15.
Next, referring to FIG. 2 which illustrates that a second opening is formed in the first BARC layer 15 and a third opening is formed in the DARC layer 14 after an etching process has been performed along the first opening. The third opening has a depth smaller than a thickness of the DARC layer 14. The second and third openings are interconnected.
After that, referring to FIG. 3 which illustrates the structure after the first photoresist layer 16 and the first BARC layer 15 are removed.
Next, referring to FIG. 4 which illustrates the structure after a second BARC layer 17 and a second photoresist layer 18 are formed on the DARC layer 14. The second BARC layer 17 fills the third opening. A fourth opening is formed in the second photoresist layer 18, exposing the underlying second BARC layer 17.
After that, referring to FIG. 5 which illustrates that a fifth opening is formed in the second BARC layer 17 and a sixth opening is formed in the DARC layer 14 after an etching process has been performed along the fourth opening. The sixth opening has a depth smaller than the thickness of the DARC layer 14. The fifth and sixth openings are interconnected.
Next, referring to FIG. 6 which illustrates the structure after the second BARC layer 17 and the second photoresist layer 18 are removed.
After that, referring to FIG. 7 which illustrates that a first contact hole and a second contact hole are formed after etching processes have been performed to the third dielectric layer 13, the second dielectric layer 12 and the first dielectric layer 11 along the respective third opening and sixth opening. The first contact hole is situated below the third opening while the second contact hole is situated below the sixth opening.
At last, referring to FIG. 8 which illustrates the structure after the DARC layer 14 is removed.
However, in practice, after the performance of the step shown in FIG. 4, namely after forming the fourth opening in the second photoresist layer 18, if the fourth opening does not achieve a desired diameter, a repetition process, namely removing the second photoresist layer 18 and the second BARC layer 17 by etching and repeating the step shown in FIG. 4, must be carried out until the diameter of the newly formed fourth opening meets the requirement. In such repetition process, the third opening may be damaged and widened during the etching step for removing the second BARC layer 17, which will lead to a too large diameter of the first contact hole that will be formed under the third opening. Therefore, there is a need to improve the stability of the contact-hole formation process of the prior art.